This invention relates to a novel apparatus capable of measuring distribution of a shot stream.
During a shot peening process, a stream of shot (i.e., particles), traveling at a high velocity, is directed at a workpiece surface. The shot is directed at the workpiece so as to cause plastic deformation of the workpiece surface, which often is a metal surface. Although this process may be applied for other purposes, the shot peening process generally is used to increase fatigue strength of the workpiece.
Two key drivers govern the shot peening process. They are intensity, the impact energy of individual shot particles, and coverage, the way individual strikes add to fully cover the surface with a compressive residual stress layer.
Under normal shot peening conditions, it is desirable to expose the substrate or work surface to the shot stream for a sufficient time to achieve 100% surface coverage, or sufficient exposure to fully cover the surface with impact dimples. Insufficient coverage has obvious adverse consequences. If the local compressive zones, due to individual shot strikes, do not merge into a continuum that fully covers the surface with a compressive layer, locally unprotected regions will exist which may cause some regions of the surface to remain in tension and become crack initiation sites.
On the other hand, shot peening related damage may also be caused by excessive shot peening, leading to immoderate cold work. The cold work sustained by the material is a function of both coverage and intensity, as well as shot size. To minimize damage age due to excessive cold work, or prevent under worked parts form entering service, it is desirable to measure the coverage.
For uniform flat surfaces it is relatively easy to set and control the exposure time needed to maintain the desired coverage condition. However, on surfaces with features, such as inside corners where reflections cause multiple strikes, it may be difficult to avoid excessive coverage, in some cases to the point where surface distress may occur.
Certain measurement techniques have been used in conjunction with the shot peening process, however, such prior techniques have been inadequate to conveniently and inexpensively provide an indication of the quality of a shot peening technique. The general absence of simple and inexpensive techniques to measure the quality of shot peening inhibits one's confidence that consistent shot peening results may be obtained.
It is apparent from the above that there exists a need in the art for a quick, inexpensive, and relatively accurate apparatus for measuring shot stream particle density distributions in order to evaluate the coverage on a Substrate surface in a manner which provides a full and complete analysis of the coverage. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.